In general, titanium alloys are used for automobile parts, and in particular, β titanium alloys are commonly used for suspension springs in which high strength is required, valve springs of engines, and suspension springs for motorcycles. The titanium alloys that are generally classified as β type have superior cold workability and are relatively easily provided with high strength by heat treatment. The β titanium alloys are usually made by solution treating a β phase material, which is stable at high temperatures, so as to have a metastabilized β phase at room temperature. Therefore, β phase stabilizing elements such as V and Mo, which are expensive, and Cr, must be added to the β titanium alloys in large amounts. Accordingly, demands for titanium alloy parts made of inexpensive materials with strengths comparable to that of the β titanium alloys have been increasing.
In the β titanium alloys, the strength can be increased by a heat treatment such as an α phase precipitation hardening treatment. However, for mechanical parts, fatigue strength is important in view of practical use. The β titanium alloys break by cracks that are generated within the precipitated α phase grain or that are generated at a boundary between the α phase and the β phase. These cracks are thought to occur due to difference in elastic strain between the α phase and the β phase. Therefore, in a structure such as in the β titanium alloys that are strengthened by precipitating the α phase from the β matrix phase in an aging treatment, static strength is superior, but improvement of the fatigue strength is limited. On the other hand, titanium alloys of a near α type and an α+β type contain small amounts of the expensive β phase stabilizing elements and small amount of the β phase that is easy to deform and has low strength. Therefore, for the above reasons, these titanium alloys are anticipated to be usable in automobile parts in view of production costs and strengths.
As disclosed in, for example, Japanese Patent No. 3789852, a Ti-6Al-4V (mass %) alloy, which is typical as the α+β type, has a good balance of mechanical characteristics such as strength, ductility, and toughness. Therefore, the amount of production of this alloy accounts for approximately 70% of the total amount of production of titanium alloys, and the penetration rate of this alloy is high. Accordingly, the Ti-6Al-4V alloy is inexpensive and has small variations in the compositions and the material strength.
The mechanical characteristics of the Ti-6Al-4V alloy mainly depend on a structure shape. That is, the characteristics and the strength depend on whether the structure is an equiaxed structure, an acicular structure, or a bimodal structure. In general, the equiaxed structure is superior in strength, elongation, resistance to fatigue crack initiation, and plastic workability. The acicular structure is superior in creep resistance, fracture toughness, and crack growth resistance. The bimodal structure has the advantages of both the equiaxed structure and the acicular structure.
The conventional structural control of the Ti-6Al-4V alloy by working is mainly performed by hot working in a temperature range in which the β phase or the α+β phase is stable. In this case, a starting structure before the hot working is a structure of equiaxed α+β phase or of acicular α+β phase. The present inventors had an idea that refining of the crystal grains may be effective for obtaining a material which has superior workability for part shapes and has high strength. Then, the present inventors experimented with various thermomethanical treatments on a structure of the equiaxed α+β phase or of the acicular α+β phase as a starting structure. As a result, however, the grain sizes of the α crystals were on the order of micrometers even at the smallest, or the structures were mixed with coarsened grains and were not uniform. Moreover, the structures could be structures other than the equiaxed structure. Accordingly, superior workability for part shapes and good mechanical characteristics were not anticipated for the structures.